The present invention relates to semiconductor integrated circuits (ICs), and more particularly to a back-end-of-the-line (BEOL) interconnect structure.
The continuous scaling to smaller dimensions and the introduction of low-k dielectrics in copper interconnects have caused reliability issues to become a greater concern in addition to increasing process complexity. Chemical Mechanical Polish (CMP) residues may cause wire to wire electrical shorts. This problem is becoming more serious as the wire to wire spacing is decreased.
It has been observed that post CMP Cu scratches and/or residue between interconnects are a major cause of dielectric breakdown failure. This dielectric breakdown failure is a reliability issue which is getting worse as IC critical dimensions continue to scale down.
In semiconductor interconnect structures, electromigration (EM) has been identified as a metal failure mechanism. EM is a serious reliability concern for very large scale integrated (VLSI) circuits. Voids are created inside the metal conductor of an interconnect structure due to metal ion movement caused by the high density of current flow.
Although the fast diffusion path in metal interconnects varies depending on the overall integration scheme and materials used for chip fabrication, it has been observed that metal atoms, such as Cu atoms, transported along the metal/post planarized dielectric cap interface play an important role on the EM lifetime projection. The EM initial voids first nucleate at the metal/dielectric cap interface and then grow in the direction of the bottom of the interconnect. This can result in a circuit dead opening.
Metal cap interconnect systems are known to have better electromigration resistance as compared to conventional dielectric cap interconnect systems. The Cu/metal interface has better adhesion strength than the Cu/dielectric interface. This results in better electromigration resistance in the Cu/metal capping layer system. Selective Co alloy deposition on Cu interconnects has been demonstrated to have higher electromigration resistance than the Cu/dielectric capping layer system. A problem with selective Co deposition is observed Co residues between interconnects. Metallic residues from metal cap deposition processes have caused wire to wire shorts and are a problem with implementing a metal cap interconnect system for high volume manufacturing.
In addition to the reliability issues caused by the metal residues, either from CMP scratches or metal cap deposition, weak mechanical strength at the cap/dielectric/barrier interface also leads to Cu diffusion into the dielectric which causes circuit reliability degradation.
FIG. 1 is a cross sectional view of a conventional interconnect structure illustrating the mechanically weak interface. Specifically, the interconnect structure 10 shown in FIG. 1 comprises a dielectric material 12 having a dielectric constant of approximately 4.0 or less. Embedded within the dielectric material 12 is a conductive material 16, preferably Cu, that is separated from the dielectric material 12 by a diffusion barrier 14. A capping layer 18 is located atop the dielectric material 12 as well as upper portions of the diffusion barrier 14 and on the conductive material 16. The conventional interconnect structure 10 is mechanically weak at the triple interface that is formed between the capping layer 18/dielectric material 12/diffusion barrier 14. The mechanically weak interface is denoted by reference numeral 20.
In addition to the reliability issues caused by the mechanically weak interface, conductive material 16 eventually diffuses into the dielectric material 12 and causes reliability degradation of the circuit. The diffusion of conductive material to the dielectric material 12 occurs readily in the absence of the diffusion barrier 14 under the influence of an electrical field. Similarly, it has also been observed that the conductive material 16, particularly Cu ions, can diffuse into the dielectric material 12 along the conductive material 16/capping layer 18 interface under normal circuit operations.
An object of the present invention is to provide a novel interconnect structure with higher mechanical strength at the cap/dielectric/barrier interface and enhanced circuit reliability.
Another object of the present invention is to provide a novel interconnect structure with high dielectric breakdown resistance. Another object of the present invention is to provide fabrication methods which are compatible with current BEOL processes without adding extra masking steps or increased cost.